1. Technical Field
The present disclosure relates to a microelectromechanical gyroscope with a self-calibration function and to a method for calibrating a microelectromechanical gyroscope.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has become increasingly widespread in various sectors of technology and has yielded encouraging results especially in the production of inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS of this type are usually based upon microelectromechanical structures comprising at least one mass connected to a fixed body (stator) by springs and movable with respect to the stator according to pre-set degrees of freedom. The movable mass and the stator are capacitively coupled through a plurality of respective comb-fingered electrodes facing one another so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors, whence it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and hence to the force applied. Instead, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass to set it in motion. Furthermore, to obtain electromechanical oscillators, the frequency response of the inertial MEMS structures is exploited, which is typically of a second-order lowpass type, with a resonance frequency.
In particular, MEMS gyroscopes have a more complex electromechanical structure, which comprises two masses that are movable with respect to the stator and coupled together so as to have a relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to driving and is kept in oscillation at the resonance frequency. The other mass is drawn along in the oscillatory (translational or rotational) motion and, in the event of rotation of the microstructure with respect to a pre-set gyroscopic axis with an angular velocity, it is subject to a Coriolis force proportional to the angular velocity itself. In practice, the mass that is drawn along, which is capacitively coupled to the fixed body through electrodes, as likewise the driving mass, operates as an accelerometer that enables detection of the Coriolis force and acceleration and hence tracing back to the angular velocity.
As already mentioned, the structure of MEMS gyroscopes is rather complex and, among other things, the exact configuration of the masses and of the electrodes necessary for driving and sensing affects the capacitive coupling. In practice, inevitable imperfections due to process spread result in systematic errors that alter the results of the measurements. For instance, a defect in the elastic suspension elements that constrain the masses to the stator can cause a displacement with respect to the theoretical resting position and hence an unbalancing of the capacitances. The systematic unbalancing due to the process spread (offset) carries, however, a considerable weight and as a rule has a much greater effect than the unbalancing caused by the measured quantity (in particular, an angular velocity). Even though the offset is translated in frequency by demodulation and subsequently filtered, the dynamics of the components that intervene in the processing prior to filtering is severely limited. In other words, the dynamics is almost saturated by the offset, and the fraction available for the signal is consequently compressed. Furthermore, even when the dynamics available for the useful signal is sufficient, the demodulation and filtering cannot cause total suppression of the contribution of offset, which presents in a form similar to the noise on the output. Also this aspect may seriously limit the use of microelectromechanical gyroscopes, especially for applications where an extremely low level of noise is required.
For this reason, gyroscopes are calibrated in the factory using auxiliary capacitances having a variable value, which is selected so as to compensate for the offsets.
This solution is not, however, satisfactory, because the offsets linked to the structure, especially due to capacitive unbalancing, are not stable and depend to a large extent upon the conditions, in particular the temperature. Even just the thermal stresses during the steps of soldering of the devices can cause important drifts and annuls the effect of calibration. In the same way, variations of temperature in use with respect to the calibration conditions may modify the offset and alter the measurements, introducing systematic errors. Another factor that can affect the offset to a considerable extent is ageing.